Geomechanical Challenges and Well Stability Related to Production from Hydrate Deposits

The geomechanical response of HBS in general, and potential well instability and casing deformation in particular, are serious concerns that need to be addressed and understood before gas production from hydrate deposits can be developed in ear­nest. Deposits that are suitable targets for production often involve poorly consoli­dated sediments that are usually characterized by limited shear strength. The dissociation of the solid hydrates (a strong cementing agent) can degrade the struc­tural strength of the HBS, which is further exacerbated by the evolution of expand­ing gas zone, progressive transfer of loads from the hydrate to the sediments, and subsidence. The problem is at its highest intensity in the vicinity of the wellbore where the largest changes are concentrated, and is further complicated by produc­tion-induced changes in the reservoir pressure and temperature. These can significantly alter the local stress and strain fields, with direct consequences on the wellbore stability, the flow and fluid properties of the system, the potential for coproduction of solid particles, and the overall gas production.

Recent coupled flow-geomechanical simulations investigated wellbore and res­ervoir instability during depressurization-based production from known oceanic and permafrost-related hydrate deposits [169-171]. The modeling results show that geomechanical responses during depressurization-based gas production are driven by the reservoir-wide pressure depletion, DP, which is in turn controlled by the production rate and pressure decline at the well. The depressurization of the reser­voir causes vertical compaction and stress changes, which in most cases will increase the shear stresses within the reservoir, which in turn can induce shear failure. The effect of pressure depletion on subsidence and stress during gas production from an oceanic Class 2 deposit in Fig. 16 shows that subsidence is proportional to the mag­nitude of DP, and depends on the elastoplastic properties of the HBS. In general, subsidence will be much larger in oceanic HBS because of much larger DP than in the case of permafrost-associated deposits. For the example in Fig. 16, the subsid­ence is about 2.5 m, and is a result of compaction in the hydrate-free, relatively soft, zone of mobile water. In the case of production from permafrost deposits at Mallik and Mt. Elbert, DP was limited to a few MPa, resulting in a subsidence of only a few centimeters and a compaction strain of less than 1% [171]. Subsidence in this case is also reduced as a result of a relatively stiff permafrost overburden. A general observation is that subsidence occurs uniformly over a large lateral distance from the well, and may thus be less of a hazard to overlying structures.

Fig. 16 Numerical simulation results of depressurization-based gas production from a class 2 oceanic hydrate deposit using a vertical production well: (a) production rates (CH4, water, and total rates), (b) pressure and temperature, (c) vertical displacement (subsidence), and (d) maximum and minimum principal stresses [ 127 ]

Rutqvist and Moridis [170] showed that the likelihood of inducing shear failure in the reservoir (a) depends on the initial stress field and the Poisson’s ratio of the host sediment, and (b) it is higher in the case of an oceanic HBS. If the stress field is initially near critical stress for shear failure, even a small pressure decline could suffice to trigger shear failure in parts of the dissociated reservoir, thus enhancing subsidence and sand production.

Stress changes and associated strain resulting from depressurization strongly affect well stability and the load on the well casing [168]. In vertical wells, the pres­sure depletion will generally unload the formation uniformly in a plane normal to the axis of the well, and the load on the well casing will decrease. In horizontal wells, vertical compaction of the formation acting against the upper part of the relatively stiff well casing will likely cause shear failure in the formation in that area. Such shearing of the formation can break the bonds between particles, resulting in sand production and creation of cavities around the wellbore. Several studies indicate the difficulty of avoiding shear failure in the formation around the production intervals of the wells, and highlight the need for appropriate engineering measures to prevent solid production [127].

HBS stress changes and the vertical compaction can be substantial in oceanic HBS. Moreover, formation failure may also occur in the form of pore collapse, in which the mean effective stresses increase so much that inelastic grains slippage and rearrange­ment occurs. Oceanic HBS are often at the highest effective stress in their geological life, which means that their pre-consolidation pressure would likely be exceeded during depressurization-induced production. Under pore collapse, f and k may be subjected to more substantial, and often irreversible, changes. Such processes and their affect on the gas production from the HBS need to be further investigated.

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